This disclosure relates to the manufacture of glass sheets such as the glass sheets used as substrates in display devices, e.g., liquid crystal displays (LCDs). More particularly, the disclosure relates to methods and apparatus for controlling the stress in, as well as the shape of, the glass ribbon from which such sheets are produced in a downdraw glass manufacturing process (e.g., the fusion downdraw process), as well as the stress in and the shape of glass sheets made from the ribbon.
Display devices are used in a variety of applications. For example, thin film transistor liquid crystal displays (TFT-LCDs) are used in notebook computers, flat panel desktop monitors, LCD televisions, and internet and communication devices, to name only a few.
Many display devices, such as TFT-LCD panels and organic light-emitting diode (OLED) panels, are made directly on flat glass sheets (glass substrates). To increase production rates and reduce costs, a typical panel manufacturing process simultaneously produces multiple panels on a single substrate or a sub-piece of a substrate. At various points in such processes, the substrate is divided into parts along cut lines.
Such cutting changes the stress distribution within the glass, specifically, the in-plane stress distribution seen when the glass is vacuumed flat. Even more particularly, the cutting relieves stresses at the cut line such that the cut edge is rendered traction free. Such stress relief in general results in changes in the vacuumed-flat shape of the glass sub-pieces, a phenomenon referred to by display manufacturers as “distortion.” Although the amount of shape change is typically quite small, in view of the pixel structures used in modern displays, the distortion resulting from cutting can be large enough to lead to substantial numbers of defective (rejected) displays. Accordingly, the distortion problem is of substantial concern to display manufacturers and specifications regarding allowable distortion as a result of cutting are challenging.
In addition to producing distortion when glass sheets are cut into sub-pieces, stress, including both residual stress frozen into the glass, which is the source of distortion, and temporary stress, which dissipates as the glass' temperature equilibrates, also affects the shape of the glass ribbon used to manufacture the glass sheets. The shape of the glass ribbon, in turn, affects such processes as sheet separation. In particular, the shape of the ribbon affects both the scoring and subsequent separation of individual sheets from the ribbon, as well as the movement of the ribbon during scoring.
In view of the foregoing, extensive efforts have been made to control the stress in, and the shape of, the glass ribbons used to produce glass sheets in downdraw glass making processes. The present disclosure identifies a source of undesirable stresses and undesirable ribbon shapes, not previously disclosed in the art, and provides methods and apparatus for reducing the adverse effects of these undesirable stresses and shapes on both the glass ribbon and on finished sheets made from the ribbon.